If you grow large enough, you'll find that you don't need to charge your users anything as charities will become your customers. Charities have huge budgets for fundraising, and would pay to be promoted as a donation option.
That's a very good point! We actually try to focus on smaller, more local charities that don't have large marketing budgets (there are some exceptions) - for instance, Rocket Dog Rescue doesn't even pay a staff, they are entirely volunteer-run. Their entire marketing strategy is unpaid, and we LOVE that, since every dollar they get is spent on helping amazing animals find homes.
These are the kind of charities we want to make sure are getting funded - the ones that make the biggest impact/dollar in a local community.